The present invention relates generally to exposure apparatuses, and more particularly to a reflection type or catoptric projection optical system that uses ultraviolet (“UV”) and extreme ultraviolet (“EUV”) light to project and expose an object, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (“LCD”), an exposure apparatus, and a device fabrication method.
Along with recent demands for smaller and lower profile electronic devices, finer semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, the design rule for mask patterns has required that an image with a size of a line and space (“L & S”) of less than 0.1 μm be extensively formed and it is expected to require circuit patterns of less than 80 nm in the near future. L & S denotes an image projected onto a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus as a typical exposure apparatus for fabricating semiconductor devices includes a projection optical system for projecting and exposing a pattern on a mask (reticle), onto a wafer. The resolution R of the projection exposure apparatus (i.e., a minimum size for a precise image transfer) can be defined using a light-source wavelength λ and the numerical aperture (“NA”) of the projection optical system as in the following equation:
                    R        =                              k            1                    ×                      λ            NA                                              (        1        )            
As the shorter the wavelength becomes and the higher the NA increases, the better the resolution becomes. The recent trend has required that the resolution be a smaller value; however it is difficult to meet this requirement using only the increased NA, and the improved resolution expects use of a shortened wavelength. Exposure light sources have currently been in transition from KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm) to F2 excimer laser (with a wavelength of approximately 157 nm). Practical use of the EUV light is being promoted as a light source.
As a shorter wavelength of light limits usable glass materials for transmitting the light, it is advantageous for the projection optical system to use reflection elements, i.e., mirrors instead of using many refraction elements, i.e., lenses. No applicable glass materials have been proposed for the EUV light as exposure light, and a projection optical system could not include any lenses. It has thus been proposed to form a catoptric reduction projection optical system only with mirrors.
A mirror in a catoptric reduction projection optical system forms a multilayer film to enhance reflected light and increase reflectance, but the smaller number of mirrors is desirable to increase reflectance of the entire optical system.
In addition, NA should be increased, for example, p to 0.2 for a wavelength of 13.5 nm, since a critical dimension (resolution) required for the EUV exposure apparatus has been smaller than the conventional values. However, it is difficult to reduce the wave front aberration in the conventional three- or four-mirror system. Accordingly, a demand has occurred to increase the number of mirrors as well as making a mirror aspheric so as to increase the degree of freedom in correcting the wave front aberration.
For example, Japanese Patent Publication Applications Nos. 2000-100694 and 2000-235144 disclose a catoptric reduction optical system that includes about six mirrors (sometimes expressed as a six-mirror system hereinafter). Japanese Patent Publication Application No. 10-90602 discloses a catoptric reduction projection optical system that uses six or eight mirrors and has NA of about 0.45. Japanese Patent Publication Application No. 2002-116382 discloses another example of a catoptric reduction projection optical system that uses eight mirrors and has NA of about 0.25.
The six-mirror system proposed in Japanese Patent Publication Application No. 2000-100694 reflects a principal ray near a vertex of a first mirror, and tends to cause large telecentricity at the object side, i.e., an angle between the principal ray incident upon the first mirror and an optical axis. Thus, an offset of an object-surface position in the optical-axis direction at the time of scan exposure would cause the magnification and distortion on the image surface to easily vary and the imaging performance to deteriorate.
Another disadvantage is that it handles NA up to about 0.16 but the six-mirror system has an insufficient degree of freedom in correcting aberration and a difficulty in handling higher NA up to about 0.5. This is because a second reflection system from an intermediate image to the image surface includes as many as four mirrors, and it becomes difficult to arrange these mirrors without interfering with light other than the reflected light as the high NA thickens a beam width in the second reflection optical system. Although a high principal-ray point in the second reflection optical system, in particular, at the third and forth mirrors might enable these mirrors to be arranged without interference, the second mirror as a concave mirror hinders the arrangement. It is conceivable that the object point is made higher to handle the higher NA, but a wider angle is incompatible with a correction of aberration as well as causing a large mirror size.
Since a minimum distance between the object surface and the mirror is as short as about 20 to 30 mm, it is difficult to maintain a space for a stage mechanism for scanning the object surface. Thus, the illumination light disadvantageously interferes with the stage mechanism when the illumination system is arranged so that it crosses the optical axis of the projection optical system.
The six-mirror catoptric projection optical system as proposed in Japanese Patent Publication Application No. 2000-235144, the minimum distance between the object surface and the mirror is so short as about 80 to 85 mm, and thus it is still difficult to maintain a space for a stage mechanism for scanning the object surface. Therefore, the illumination light disadvantageously interferes with the stage mechanism when the illumination system is arranged so that it crosses the optical axis of the projection optical system.
The six-mirror has an insufficient degree of freedom in correcting aberration, and cannot provide sufficient performance, even when seeking for higher NA, e.g., about 0.5. In addition, in order to avoid interference of light at the fifth and sixth mirrors in the higher NA, an angle of a principal ray relative to the optical axis must be increases between the fourth and fifth mirrors and an effective diameter of a fourth mirror disadvantageously increases.
The projection optical system proposed in Japanese Patent Publication Application No. 10-90602 does not clearly describe its performance and is an optical system for UV light having a wavelength between 100 to 300 nm. Therefore, aberration must be reduced down to about 1/10 in order to use this optical system for the EUV light, which is difficult in correcting aberration.
The six-mirror system has problems similar to those associated with Japanese Patent Publication Application No. 2000-235144, and the eight-mirror system shown in FIG. 11 is also disadvantageous. Although this reference discloses an optical system having NA of 0.5, an actual measurement reveals that NA is about 0.4 and reflective surfaces 1100 and 1200 have such a large incident angle as about 56° and 54°, respectively. This is because a reflective surface 1300 forms a concave mirror, a reflective surface 1400 forms a convex mirror, and an aperture stop 1500 is formed between them. Since light from an object is greatly reflected on the reflective surface 1300 of positive power in an optical-axis direction and then reflected, through the aperture stop 1500, on the reflective surface 1400 of negative power at a portion below the optical axis, an incident angle upon the reflective surface 1100 as a convex mirror. Here, FIG. 11 is a schematic sectional view of a conventional eight-mirror catoptric projection optical system 1000.
In addition, distances X and Y become short, because the light from a reflective surface 1600 to a reflective surface 1700 is reflected so that it approaches to the optical axis and a reflective surface 1200 is located closer to the image surface than a reflective surface 1800. Therefore, reflections with strong power in small space would increase an incident angle upon the reflective surface 1200. Use of the EUV light with a large incident angle is incompatible with an angular characteristic of a multilayer film layered on the mirror, causing reduced light intensity, non-uniform light intensity, and non-uniform resolution on a wafer surface. Moreover, it is doubtful that the optical system has performance of resolution limit as a whole due to increased telecentricity at the side of wafer.
The projection optical system proposed in Japanese Patent Publication Application No. 2002-116382 enables an eight-mirror system to have NA of about 0.25. However, this is also disadvantageous in that a configuration generally arranges the fourth mirror closest to a mask side, and thus a diameter of the fourth mirror easily increases. In addition, since beams cross in the optical path in FIGS. 2, 6, 8 and 10, and higher NA would cause interference between the light and the mirror(s).
In other words, a catoptric projection optical system having high NA suitable for the EUV light, e.g., greater than about 0.25 or about 0.3 to 0.5 has not yet been proposed.